Sabah Shawkat Cabinet of Structural Engineering 2017

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3.9 Concrete Foundations A foundation is a integral part of the structure which transfer the load of the superstructure to the soil without excessive settlement.

1 3.9 Concrete Foundations A foundation is a integral part of the structure which transfer the load of the superstructure to the soil without excessive settlement. A foundation is that member which provides support for the structure and it's loads. It also provides a means by which forces or movements within the ground can be resisted by the building. In some cases, foundation elements can perform a number of functions: for example, a diaphragm wall forming part of a basement will usually be designed to carry loading from the superstructure. If new foundations are placed close to those of an existing building, the loading on the ground will increase and movements to the existing building may occur. When an excavation is made, the stability of adjacent buildings may be threatened unless the excavation is adequately supported. This is particularly important with sands and gravels which derive their support from lateral restraint. The choice of foundation type or the type of foundation selected for a particular structure is influenced by the following factors: 1. The imposed loads or deformations, the magnitude of the external loads 2. Ground conditions, the strength and compressibility of the various soil data 3. The position of the water table 4. Economics 5. Buildability, and the depth of foundations of adjacent structures 6. Durability. Figure: Foundations of tall building

2 An essential requirement in foundations is the evaluation of the load which a structure can safely bear. The types of foundation generally adopted for building and structures are spread (pad), strip, balanced and cantilever or combined footings, raft and pile foundations. For example, strip footings are usually chosen for buildings in which relatively small loads are carried mainly on walls. When the spread footings occupy more than half the area covered by the structure and where differential settlement on poor soil is likely to occur a raft foundation is found to be more economical. Pad footings, piles or pile groups are more appropriate when the structural loads are carried by columns. If differential settlements must be tightly controlled, shallow strip or pad footings (except on rock or dense sand) will probably be inadequate so stiffer surface rafts or deeper foundations may have to be considered as alternatives. This type of foundation viewed as the inverse of a one-storey beam, slab and column system. The slab rests on soil carrying the load from the beam/column system which itself transmits the loads from the superstructure. Figure: Types of foundations These are generally supporting columns and may be square or rectangular in plan and in section, they may be of the slab, stepped or sloping type. The stepped footing results in a better distribution of load than a slab footing. A sloped footing is more economical although constructional problems are associated with the sloping surface. The isolated spread footing in plan concrete has the advantage that the column load is transferred to the soil through dispersion in the footing. In reinforced concrete footings, i.e. pads, the slab is treated as an inverted cantilever bearing the soil pressure and supported by the column. Where a two-way footing is provided it must be reinforced in two directions of the bending with bars of steel placed in the bottom of the pad parallel to its sides.

3 Foundations under walls or under closely spaced rows of columns sometimes require a specific type of foundation, such as cantilever and balanced footings and strip footings. Pad footing Square or rectangular footing supporting a single column. Strip footing Long footing supporting a continuous wall. Combined footing Footing supporting two or more columns. Balanced footing Footing supporting two columns, one of which lies at or near one end. Raft Foundation supporting a number of columns or loadbearing walls so as to transmit approximately uniform loading to the soil. Pile cap Foundation in the form of a pad, strip, combined or balanced footing in which the forces are transmitted to the soil through a system of piles. The plan area of the foundation should be proportioned on the following assumptions: a. All forces are transmitted to the soil without exceeding the allowable bearing pressure b. When the foundation is axially loaded, the reactions to design loads are uniformly distributed per unit area or per pile. A foundation may be treated as axially loaded if the eccentricity does not exceed 0.02 times the length in that direction c. When the foundation is eccentrically loaded, the reactions vary linearly across the footing or across the pile system. Footings should generally be so proportioned that zero pressure occurs only at one edge. It should be noted that eccentricity of load can arise in two ways: the columns being located eccentrically on the foundation; and/or the column transmitting a moment to the foundation. Both should be taken into account and combined to give the maximum eccentricity. d. All parts of a footing in contact with the soil should be included in the assessment of contact pressure e. It is preferable to maintain a reasonably similar pressure under all foundations to avoid significant differential settlement.

4 3.9.1 Shallow Foundations A shallow foundation distributes loads from the building into the upper layers of the ground. Shallow foundations are susceptible to any seismic effect that changes the ground contour, such as settlement or lateral movement. Such foundations are suitable when these upper soil layers have sufficient strength ( bearing capacity ) to carry the load with an acceptable margin of safety and tolerable settlement over the design life. The different types of shallow foundation are: a) Strip footing b) Spread or isolated footing c) Combined footing Strap or cantilever footing d) Mat or raft Foundation.

5 Punching in Spread Footing Figure: Shallow Foundations Figure: Spread Footing Design of Reinforcement

6 Figure: Figure:

2 Strap Footing It consists of two isolated footings connected with a structural strap or a lever, as

7 Figure: Figure: Strap Footing It consists of two isolated footings connected with a structural strap or a lever, as shown in figure The strap connects the footing such that they behave as one unit. The strap simply acts as a connecting beam. A strap footing is more economical than a combined footing when the allowable soil pressure is relatively high and distance between the columns is large.

8 Figure: Figure: Figure:

3.9.3 Combined Footing It supports two columns as shown in fig. 3.9.3-1. It is used when the two columns are so close to each other that their individual footings would overlap.

3-1 Combined Footing 3.9.4 Strip/continuous footings A strip footing is another type of spread footing which is provided for a load bearing wall.

9 3.9.3 Combined Footing It supports two columns as shown in fig It is used when the two columns are so close to each other that their individual footings would overlap. A combine footing may be rectangular or trapezoidal in plan. Trapezoidal footing is provided when the load on one of the columns is larger than the other column. Figure: Combined Footing Strip/continuous footings A strip footing is another type of spread footing which is provided for a load bearing wall. A strip footing can also be provided for a row of columns which are so closely spaced that their spread footings overlap or nearly touch each other. In such a cases, it is more economical to provide a strip footing than to provide a number of spread footings in one line. A strip footing is also known as continuous footing.

10 Figure: A traditional strip foundation consists of a minimum thickness of 150 mm of concrete placed in a trench, typically m wide. Reinforcement can be added if a wider strip is required to bridge over soft spots at movement joints or changes in founding strata. Figure: Mat or Raft footings It is a large slab supporting a number of columns and walls under entire structure or a large part of the structure. A mat is required when the allowable soil pressure is low or where the columns and walls are so close that individual footings would overlap or nearly touch each other. Mat foundations are useful in reducing the differential settlements on non-homogeneous soils or where there is large variation in the loads on individual columns. Figure:

Figure: 3.9.5-2 3.9.6 Pile foundations Deep foundations are used when the soil at foundation level is inadequate to support the imposed loads with the required settlement criterion.

11 Figure: Pile foundations Deep foundations are used when the soil at foundation level is inadequate to support the imposed loads with the required settlement criterion. Where the bearing capacity of the soil is poor or the imposed load are very heavy, piles, which may be square, circular or other shapes are used for foundations. If no soil layer is available, the pile is driven to a depth such that the load is supported through the surface friction of the pile. The piles can be precast or cast in situ. Deep foundations act by transferring loads down to competent soil at depth and/or by carrying loading by frictional forces acting on the vertical face of the pile. Diaphragm walls, contiguous bored piles and secant piling methods are covered later in this chapter. Short-bored piles have been used on difficult ground for low-rise construction for many years. They can be designed to carry loads with limited settlements, or to reduce total or differential settlements. They can have bases that are flat, pointed or bulbous, and shafts that are vertical or raked. In some circumstances, piles can be constructed of other materials, such as timber or plastics. Piled walls or sheet piles are used to resist lateral movements, such as in forming a basement.

The piling technique used to install the piles will be determined by the ground conditions, loading requirements for the final pile as well as other factors such as access or proximity to other

6-1 Figure: 3.9.6-2 Piles are individual columns, generally constructed of concrete or steel, that support loading through a combination of friction on the pile shaft and end-bearing on the pile toe.

12 The piling technique used to install the piles will be determined by the ground conditions, loading requirements for the final pile as well as other factors such as access or proximity to other buildings and the need for noise reduction. Pile types There are two basic types of piles: cast-in-place (or replacement) piles and driven (or displacement) piles. Figure: Figure: Piles are individual columns, generally constructed of concrete or steel, that support loading through a combination of friction on the pile shaft and end-bearing on the pile toe. The distribution of load carried by each mechanism is a function of soil type, pile type and settlement. They can also be used to resist imposed loading caused by the movement of the surrounding soil, such as vertical movements of shrinking and swelling soils. Piles can be installed vertically or may be raked to support different loading configurations.

reinforcement should not be less than 0.0015bh in each direction.

13 Figure: All pile caps should generally be reinforced in two orthogonal directions on the top and bottom faces and the amount of reinforcement should not be less than bh in each direction. The bending moments and the reinforcement should be calculated on critical sections at the column faces, assuming that the pile loads are concentrated at the pile centres. This reinforcement should be continued past the piles and bent up vertically to provide full anchorage past the centreline of each pile. Figure:

14 Figure: Figure:

15 Figure : collapse of unbearable soil

Figure 3.9.6-4: Main reinforcement in slab foundation Example 3.

16 Figure : Main reinforcement in slab foundation Example 3.9-1: Assessment of slab foundation to punching Depth of the reinforced slab foundation: h d 80cm Tensile strength of concrete: f ctm 0.9MPa Width of column: b s 50cm Height of column: h s 40cm Design strength of reinforcement: f yd 375MPa Figure: Perimeter of critical cross-section: h s h d 2 u cr b s h d u cr 5m

Shearing force carrying by concrete: Q bu 0.42h d f ctm u cr Q bu 1512 kn P 1 2700kN P P 1 Q bu P 1188kN Required surface area of reinforcement to punching: A sb P A sb 0.00368372m 2 0.

17 Shearing force carrying by concrete: Q bu 0.42h d f ctm u cr Q bu 1512 kn P kN P P 1 Q bu P 1188kN Required surface area of reinforcement to punching: A sb P A sb m f yd Reinforcement diameter: 2 25mm A s1 A s m 2 4 Number of profiles: A sb n n Q 0.42h d f ctm u cr Q 1512kN A s1 Figure: Data of rolled I profiles: I28 A mm 2 J1y mm 4 h 1 280mm

18 b 1 119mm b 2 119mm b 3 119mm I34 A mm 2 J1y mm 4 h 1 340mm b 1 137mm b 2 137mm b 3 137mm I38 A mm 2 J1y mm 4 h 1 380mm b 1 149mm b 2 149mm b 3 149mm h 2 20mm h 3 20mm L 1.45m P p 1 p kn M p L M mkN 8 3 b 2 h 3 2 b A 2 b 2 h 2 A 3 b 3 h 3 J 2 3 h 3 J h 1 h 2 h 3 A 1 h 2 A 2 A 3 h 1 h e 2 e m A 1 A 2 A 3 H h 1 h 2 h 3 H 0.42m e 1 H e 2 e m h 1 h 2 a 1 h 2 e 2 a 2 e 2 a 3 H h 3 e 2 a 1 0 m The total moment of inertia of composite section: J J1y J 2 J 3 A 1 a 1 A 2 a 2 A 3 a 3 J m 4 J J1y

Figure: 3.9.1-3 Section modulus: W d J W d 0.00227904m 3 J W h W h 0.

19 Figure: Section modulus: W d J W d m 3 J W h W h m 3 e 1 Stress control: d h M d 70.86MPa s 210MPa W d M h MPa s 210MPa W h e 2

20 Example 3.9-2: Determination of the design bearing capacity of the soil at depth dp =1,5 m Soli classification F6: c ef c ef 16kPa ef 21deg c d 2 d ef m Bearing coefficient of the soil: N d tan 45deg d 2 2 e tan d N b 1.5 N d 1 tan d N c 2 m ef ef 4deg 1 21 kn m Base area of the footing: Width: bp = 1 m Length: Lp = 6 m Coefficient of the shape of the footing: s c b p b p s d 1 sin d s b b p d c l p l p l p d p b p d p d d sin2 d i d 1 i c 1 i b 1 b p Design bearing capacity of the soil: R d c d N c s c d c i c 1 d p b p N d s d d d i d 2 N b s b d b i b R d k 2

Types of Foundations

Shallow Foundations Types of Foundations Foundations can be classified to two major categories: Shallow. Deep. 1 Introduction If the soil stratum is suitable for supporting the structural loads from the